Superheterodyne receiver



Jan. 13, 1942. F, R ET L 2,270,023

SUPERHETERODYNE RECEIVER Fil ed March 1-, 1939 2 Sheets-Sheet 2 Fig.3

El -AMPLIFIER I MIXER 1 QNDM/XER I. F. AMPLIFIER 2 5 9 l 3 L r I 14 I 7- 1 J BAND PA'SS LAAMpL/F/ER FILTER VAR/ABLE OSCILLATOR FIXED OSCILLA TOR RE AMPLIFIER 1 M/XER ICE AMPL/F/ER Z MIXER [.F AMPLIFIER BAND PASS\E LI FILTER 8 9/0 5 RD Z Q COA R85 CONTROL plsclzl m m q AFC DISCR/M/NA TOR NETWORK NETWORK AFC CONTROL TUBE VA R/ABLE OSCILLATOR 1 72 1 FIXED OSCILLA TOR INVENTORS JOHN FORREST RAMSAY ATTORNEY Patented Jan. 13, 1942 SUPERHETERODYNE RECEIVER John Forrest Ramsay, Chelmsford, and Arthur Leonard Oliver, Wimbledon Park, London, England,

assignors to Radio Corporation of America, a corporation of Delaware Application March 1, 1939, Serial No. 259,126 In Great Britain March 4, 1938 6 Claims.

This invention relates to superheterodyne receivers.

Practical considerations in superheterodyne receiver design have resulted in the adoption of intermediate frequencies (I. F.) of more or less conventional fairly low values, two typical practical values being 110 and 450 k. 0. per second. There are also well known advantages in adopt ing as the variable tunable oscillator of a superheterodyne receiver an oscillator of relatively high frequency, e. g. one covering a range of about 2 to 4 megacycles.

It is known to satisfy these requirements by providing two different local oscillators and two different I. F. amplifiers in a superheterodyne receiver, the first local oscillator changing the receiver wave frequency to the first I. F. and the second local oscillator changing the first I. F. to the second I. F. This proposal, however, necessarily involves considerable increase in the complexity and cost of the receiver and moreover offers some disadvantages in the case of a receiver fitted with automatic frequency control (A. F. C.) for if, as has hitherto been proposed, the A. F. C. system operates to control the frequency of the second oscillator only, although the first I. F. is thus automatically centred at the mid-band frequency of the second I. F. amplifier, unless the two local oscillators are very accurately aligned the received frequency will be in effect displaced from the central frequency of the first I..F. amplifier.

The main object of the present invention in its broadest aspect is to provide a superheterodyne receiver wherein the requirements of using an I. F. of conventional low value and at the same time employing a variable tunable local oscillator of high frequency may be satisfied in a simple manner.

, An important subordinate object of. the invention is to provide a superheterodyne receiver wherein an A. F. C. system operates to control the frequency of an oscillator, which does not have to be manually varied in tuning but does not present the disadvantage of possible misalignment as above described. It will be. appreciated that the design of an A. F. C. system will be considerably simplified if that system has merely to vary, by relatively small amounts, the frequency of an oscillator which is not otherwise required to be variable.

. According to this invention in its broadest aspect a superheterodyne receiver is characterised in that the received signal wave frequency is beaten down to a predetermined I. F. by local oscillations which are obtained by mixing together oscillations from two local oscillators only one of which is continuously variable for manual tuning purposes.

According to an important subordinate feature of the invention .a receiver in accordance with the main feature of this invention is fitted with an A. F. C. system which operates to control the frequency of that one of the two local oscillators which is not subjected to continuously variable manual tuning control.

A preferred form of superheterodyne receiver embodying both the above mentioned features of invention will first be described in general terms, practical frequency values being given in order to simplify'the description. It is to be understood, however, that the invention is not limited to the adoption of the particular values of frequency to be given.

In the drawings, Fig. 1 represents a block diagram of so much of a superheterodyne receiver necessary for a proper understanding of the invention; Fig. 2 is a preferred and detailed circuit of a portion of the system shown in Fig. 1; Fig. 3 is a modified form of the system shown in Fig. 1, and Fig. 4 discloses the system of Fig. 3 with the application of fine and coarse A. F. C. controls.

, Referring to Figure l signals picked up on a receiving aerial I are applied to the usual signal tuning means, for example the tuning circuit of a pre-selection radio frequency amplifier 2 having atuning range of to 1500 kc. The output from this pre-selection apparatus is passed to'a first mixerstage 3 to which are also fed local oscillations obtained by mixing the oscillations from two local oscillators. One of these local oscillators 4 is variably tunable in a continuous manner for manual tuning purposes and has a frequency range of, say, 2.6 to 3.95 megacyclesj This oscillator may, if desired, be remotely tuned, as indicated, by remote tuning apparatus 5. The other oscillator 6, hereinafter termed the fixed oscillator, isnot manually tunable. but its oscillatory frequency is arranged to be varied automatically over a required small range by an A. F..C. system. The (normal) frequency of the fixed oscillator 6 is in the present example, 2 megacycles. The term fixed is used for this oscillator because, although the frequency of the said oscillator is not actually fixed, it is not continuously variable manually and the said term, though not strictly accurate,-is sufficient to distinguish the oscillator in question from the other oscillator 4. The outputs from the two oscillators are fed to a local oscillation mixer stage I and the resultant output from this stage is passed through a band pass filter circuit 8 having a pass range of 600 to 1950 kc. to the first mixer stage 3. In place of a band pass filter a tunable circuit tunable over the range 600 to 1950 kc. can be provided in order that the oscillating input to the mixer 3 may be as free from harmonics as possible. The output from the first mixer stage 3 is fed to an I. F. amplifier 9 of normal design having a mid-band frequency of 450 kc. This I. F. amplifier is followed by a demodulating detector (not shown) and a low frequency amplifier (not shown) as in the usual way. Energy from a suitable stage in the I. F. amplifier is tapped off, separately amplified if desired by an amplifier I and passed to an A. F. C. discriminator network ll' of any suitable kind known per se. The discriminator output is fed to an A. F. C. control valve l2 which operates in any manner known per se to control the frequency of the fixed local oscillator 6 within the fairly narrow limits required for A. F. C. action.

In practice the two local oscillators 4, 6, and the two mixers 3, 1, of the'abov'e receiver may be embodied in a simple circuitemploying only three valves. Such a circuit is shown in Figure 2. Referring to Figure 2 the first mixer (3 of Figure 1) is hexode 3A to whose first grid 3B the signal input from the unit 2 of Figure 1 (this is not shown in Figure 2) isapplied. In the anode circuit of this hexode is a band pass fixed tuned twin circuit arrangement 30 of normal type which feeds into the I. F. amplifier 3 of Figure 1 (not shown in Figure 2). The variably tunable local oscillator is constituted by a back coupled triode 4A and its oscillatory output is taken to the first grid 1A of the hexode section of a triode hexode 16. The third grid 1B of this hexode section is directly connected to the control grid 6A of the triode section and in the circuit of the anode 6B of thetriode section.

is included a tuned circuit BC resonantat the predetermined fixed frequency chosen for the fixed local oscillator. The A. F. C. control valve l2- (not shown in Figure, 2) operated through the discriminator network H (not shown in Figure 2) is connected in Well known manner to act as an automatically controlled variable reactance connected between terminals Tl, T2 of Figure 2 so as to form in effect part of the tuned circuit in the anode circuit of the triodesection. The tuned circuit BC isback coupled to the grid BA so that the triode section of the valve 16 operates as the fixed local oscillator 6 of Figure l and since this grid 6A is connected to the third grid 1B of the hexode section and the first grid 1A ofthe hexode section receives oscillations from the valve 4A the said. hexode section operates as the local oscillation mixer 1. Accordingly in Figure 2 the triode hexode is given the reference Hi, since it performs th functions of the elements 1 and 6 of Figure l. The anode circuit of the hexode section contains a tunable circuit 8 tunable over the range 600-1950 kc. (corresponding to the unit 8 of Figure 1) which is coupled to the third grid 3D of the hexode 3A. Thus the hexode 3A operates to produce the required I. F. since its first grid 3B receives incoming signals and its third grid 3D receives the resultant of mixing (in the hexode section) the two local oscillations.

The invention is not limited to receivers emin discrete steps in such manner that each step takenxin conjunction with the variable frequency range of the manually tunable oscillator, pro duces a range of resultant local oscillation beat frequencies which covers one of the broadcast or amateur short wave bands.

The invention is capable of application to superheterodyne receivers where--in order, for example, to obtain maximum sensitivitytwo I. F. amplifiers of different frequencies are used. In such a case the fixed local oscillator may also be used to provide heterodyning oscillations to beat the first I. F. to the second I. F. Thus the embodiment of Figure 1' may be modified as shown in Figure 3 by following the 450 kc., I. F. amplifier 9 by another mixer l3 in turn followed by an IF. amplifier 14 operating either at 2.45 megacycles or 1550 kc. Oscillations from the fixed 2 megacycle oscillator 8 are applied to the mixer I3 as well as mixed with the oscillations from the continuously tunable oscillator 4, to produce the second I. F. In such a case it is possible to provide two A. F. 0. systems, if desired both operating via a common control valve on the same oscillator, namely the fixed oscillator. One of these A. F. C. systems operates to provide fine control and the other coarse control. This is shown in Figure 4. Here the fine control system includes a fine control A. F. C. discriminator network 9H1 fed from one I. F. amplifier (the amplifier 9) and the other A. F, C. system includes a coarse discriminator A. F. C. network l4! fed from the other I. F. amplifier (the amplifier. 14) both the networks 910 and I 4 I 0 operating to control a common A. F. C. valve 12. Again the advantage is obtained that only one oscillator and that of substantially fixed frequency is controlled (the step by step variation in frequency is really only a variation to change the wave length range, the fixed" oscillator being still of fixed frequency-apart from A. F. C. control-in any one wave band covered). The order of application of the fine and coarse A, F. C. systems and the choice of the intermediate frequencies are, as will be realised, arbitrary.

A double oscillator, singl I, F. superheterodyne receiver such as that illustrated by Figure 1 is of advantage also for ultra-short wave reception. For example aband spread ultra-short wave receiver for receiving over the range60-50 megacycles requires a local oscillation range of 60.45- 5045 megacycles if the I. F, is 450' kc If the fixed oscillator frequency (from 6 of Figure 1) is 30 megacycles and the range of the oscillator 4 (Figure 1) is 20.45- to 30.45 mega'cycles, the summation of these two oscillator frequencies will give the required rangeof local oscillation frequencies for application to the mixer 3., To ensure frequency stability the. fixed oscillator 6 may be crystal controlled (the A. F. C. system of Figure 1 would, of course, .be omitted) and since the variable oscillator rangeis 20,45 to 30.45 megacycles it is more easily designed and more stable than would be a comparable oscillator with a tuning range of 50 to 60 megacycles.

The invention is also applicable to the reception of very long waves, e. g. waves up to 20,000 metres where again it offers advantages. Thus, in a system of the nature of that of Figure 1 by making the I, F. amplifier 9 of high selectivity and of a low frequency, say 12 kc. and by suitably choosing the frequency of the oscillator 6 and extending the coverage of the oscillator 4 so that the required range of low radio frequencies is produced for application to the mixer 3, long wave reception can be very eficiently achieved. The use in this way of two oscillators to produce the required local oscillations by beating with one another, avoids the need to construct a local oscillator with unduly large coils.

In the claims which follow the term fixed is employed with the rather special meaning already indicated herein to mean not subject to continuously variable manual control for tuning purposes.

We claim:

1. In the method of superheterodyne reception, the steps which consist in selecting oscillations of the received signal wave, deriving oscillations from a local source of variable frequency, deriving other oscillations from a local source of fixed frequency, first mixing the oscillations from said local sources and then mixing the resultant oscillations with the selected received signal oscillations to produce oscillations of an intermediate predetermined frequency, mixing the latter oscillations with those of the fixed frequency source to produce oscillations of another intermediate frequency and utilizing a control v voltage derived from both of said intermediate amplifier coupled to said mixer stage, a manu-' ally tunable local oscillator, a fixed frequency local oscillator, a secondmixer stage for mixing the oscillations from said oscillators, and a channel including a frequency selective network interconnected between the first and second mixer stages, characterized in that the fixed frequency oscillator and the second mixer are constituted by different sections of a double section discharge tube, each section having one or more grids and an output electrode, said fixed frequency oscillator section having a grid connected to a grid in the mixer section, said mixer section having another grid to which oscillations from the manually tunable oscillator are applied, the frequency selective network being connected to the output electrode of the mixer section and arranged to impress the resultant oscillations upon the first mixer stage.

3. In a superheterodyne receiver, a signal channel comprising a first mixer stage, an intermediate frequency amplifier, a second mixer stage and a second intermediate frequency amplifier all connected in cascade, a manually tunable local oscillator, a fixed frequency local oscillator, a third mixer stage for mixing oscillations from said oscillators, a channel including a frequency selective network interconnected between the third and first mixer stages, and a channel for supplying oscillations from the fixed frequency oscillator to the second mixer stage.

4. A receiver as claimed in claim 3 wherein a fine acting discriminator is coupled to one of the intermediate frequency amplifiers, and a coarse acting discriminator is coupled to the other of said intermediate frequency amplifiers, and a frequency control tube is coupled to both of said discriminators and to the fixed frequency oscillator.

5. In a superheterodyne receiver, a signal channel comprising at least a first mixer stage, a first intermeditae frequency amplifier, a second mixer stage and a second intermediate frequency amplifier all connected in cascade, a variably tuned local oscillator, a fixed frequency local oscillator, a third mixer stage for mixing the oscillations from both said oscillators, means for feeding the combined oscillations to the first mixer stage, a fine acting discriminator coupled to the first intermediate frequency amplifier, a coarse acting discriminator coupled to the second intermediate frequency amplifier, a frequency control tube coupled to both of said discriminators, and means for coupling the frequency control tube to the fixed oscillator.

6. In a superheterodyne receiver, a signal channel comprising at least a first mixer stage, a first intermediate frequency amplifier, a second mixer stage and a second intermediate frequency amplifier all connected in cascade, a variably tuned local oscillator, a fixed frequency local oscillator, a third mixer stage for mixing the oscillations from both said oscillators, said third mixer and the fixed oscillator being combined in one tube, means for feeding the combined oscillations to the first mixer stage, a fine acting discriminator coupled to the first intermediate frequency amplifier, a coarse acting discriminator coupled to the second intermediate frequency amplifier, a frequency control tube coupled to both of said discriminators, means for coupling the frequency control tube to the fixed oscillator, and means for feeding oscillations from the fixed oscillator to the second mixer stage.

JOHN FORREST RAMSAY, ARTHUR LEONARD OLIVER. 

